To improve thermal efficiency of gasoline internal combustion engines, lean burn is known to give enhanced thermal efficiency by reducing pumping losses and increasing ratio of specific heats. Generally speaking, lean burn is known to give low fuel consumption and low NOx emissions. There is however a limit at which an engine can be operated with a lean air/fuel mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the lean limit include improving ignitability of the mixture by enhancing the fuel preparation, for example using atomised fuel or vaporised fuel, and increasing the flame speed by introducing charge motion and turbulence in the air/fuel mixture. Finally, combustion by auto-ignition, or homogeneous charge compression ignition, has been proposed for operating an engine with very lean or diluted air/fuel mixtures.
When certain conditions are met within a homogeneous charge of lean air/fuel mixture during low load operation, homogeneous charge compression ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge, resulting in very stable power output, very clean combustion and high fuel conversion efficiency. NOx emission produced in controlled homogeneous charge compression ignition combustion is extremely low in comparison with spark ignition combustion based on propagating flame front and heterogeneous charge compression ignition combustion based on an attached diffusion flame. In the latter two cases represented by spark ignition engine and diesel engine, respectively, the burnt gas temperature is highly heterogeneous within the charge with very high local temperature values creating high NOx emission. By contrast, in controlled homogeneous charge compression ignition combustion where the combustion is uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local temperature values resulting in very low NOx emission.
Engines operating under controlled homogeneous charge compression ignition (HCCI) combustion have already been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. The high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the hot charge temperature and active fuel radicals necessary to promote homogeneous charge compression ignition in a very lean air/fuel mixture. In four-stroke engines, because the residual content is low, homogeneous charge compression ignition is more difficult to achieve, but can be induced by heating the intake air to a high temperature or by significantly increasing the compression ratio. This effect can also be achieved by retaining a part of the hot exhaust gas, or residuals, by controlling the timing of the intake and exhaust valves.
In all the above cases, the range of engine speeds and loads in which controlled homogeneous charge compression ignition combustion can be achieved is relatively narrow. The fuel used also has a significant effect on the operating range; for example, diesel and methanol fuels have wider auto-ignition ranges than gasoline fuel. A further problem is to achieve ignition at a particular time with maintained combustion stability, while avoiding engine knocking and misfiring. This is a particular problem when operating the engine under HCCI combustion at low load.
Hence there exits a need for solving the problem of controlling the combustion timing when operating the engine at low load in HCCI-mode.
Accordingly, the present invention relates to a direct fuel injection (DI) internal combustion engine preferably, but not necessarily, provided with variable valve timing (VVT), cam profile switching (CPS), and a manifold absolute pressure booster, such as a turbocharger, compressor etc. However, the general principle of the invention as claimed is also applicable to, for instance, stationary aspirating engines with fixed valve timing and a standard camshaft. Such engines are often operated at fixed speeds and loads and are not subject to the transients normally occurring in, for instance, engines for vehicles. Hence a stationary engine can be operated continuously in HCCI-mode.
Also, although the following examples relate to gasoline fuels, an engine operating according to principles of the invention can be adapted to use most commonly available fuels, such as diesel, kerosene, natural gas, and others.
A reciprocating piston is arranged in each engine cylinder whose compression action causes a mixture of air and gasoline fuel within the combustion chamber to be ignited. Gas exchange is controlled by at least one inlet valve preferably, but not necessarily, provided with variable valve timing per cylinder for admitting a combustible gas, such as air, and at least one exhaust valve preferably, but not necessarily, provided with variable valve timing per cylinder for exhausting combusted gases.
The combustion process is monitored by sensors for measuring engine knocking and combustion stability. The knock sensor can be of the piezo-electric type, which may also be used for continuous monitoring of cylinder pressure. The combustion stability sensor may be an acceleration type sensor, such as a flywheel sensor, or an ion current sensor. Alternatively, both said sensors can be replaced by a single in-cylinder piezoelectric pressure sensor. By processing the output from such a sensor, it is possible to obtain a signal representing engine knock and a signal representing engine stability.
The engine is possible to be operated on homogeneous charge compression ignition (HCCI) combustion mode. This is a combustion mode, different than conventional spark ignited (SI) combustion mode, in order to reduce fuel consumption in combination with ultra low NOx emissions. In this mode, a mixture containing fuel, air and combustion residuals is compressed with a compression ratio between 10.5 and 13 to auto ignition. The HCCI combustion has no or a very slow moving flame front, in contradiction to a SI combustion that has a moving flame front. The lack of a flame front reduces temperature and increases the heat release rate hence increases the thermal efficiency of the combustion. The stoichiometric mixture must be diluted with access air and or residuals in order to reduce the heat release rate. This reduces both pumping losses and combustion temperature hence the fuel consumption compared to an SI operated engine. The combustion residuals are captured when operating the engine with a negative valve overlap. Residuals increase the temperature of the mixture so that the auto ignition temperature is reached before piston top dead center (TDC) and dilute the mixture so that the heat release rate decreases to an acceptable level. By controlling the heat release, cycle-to-cycle variations (COV), noise and knocking combustion can be reduced. The negative valve overlap is achieved when the exhaust valve is closed before piston TDC and the inlet valve is opened after piston TDC in the gas exchange phase of the combustion, as illustrated in FIG. 2.
The acquired valve timing for the negative overlap can be achieved by using suitable fully or partially variable valve systems (VVT), and CPS, hence switching from conventional SI valve timing to HCCI valve timing with a shorter the valve opening duration and/or valve lift.
An engine according to the invention uses a gasoline internal combustion engine provided with at least one cylinder and arranged to be switched between spark ignition mode and compression ignition mode. The engine comprises a fuel injector, through which gasoline fuel is injected into a combustion chamber, for each cylinder and a fuel injection control unit that controls gasoline fuel injection quantity per combustion cycle injected through each fuel injector. Fuel injection is achieved by means of direct injection (DI) into each combustion chamber.
A split fuel injection is used having a pilot direct fuel injection before TDC during the negative valve overlap and a main (larger fuel amount) direct fuel injection after TDC of the negative valve overlap. The fuel of the pilot injection will react, forming intermediates or combustion products. This reaction can be exothermic hence heating the residuals, resulting in earlier timing of the auto ignition temperature. A spark may be sustained in HCCI mode in order to keep the spark plug from fouling and, although the gas mixture is arranged to self ignite, contribute to an increased combustion stability and avoidance of misfire.
In one preferred embodiment, an internal combustion engine is provided with at least one cylinder and includes a fuel injection system having at least one fuel injector, through which fuel is injected into a combustion chamber, for each cylinder, and a control unit for controlling the fuel injection system and a spark ignition system. The engine further includes one piston per engine cylinder whose compression action causes a mixture of air and fuel within the combustion chamber to be ignited, at least one inlet valve for admitting gas which includes fresh air into said cylinder, and at least one exhaust valve for exhausting combusted gases from said cylinder. At least one sensor for measuring an engine operation parameter may also be provided.
During compression ignition mode, the exhaust valve is arranged to be closed before top dead centre during an exhaust stroke of the piston and the intake valve is arranged to be opened after top dead centre during an induction stroke of the piston, in order achieve a negative valve overlap to retain residual exhaust gas. The control unit may be arranged to control the fuel injection system so as to perform a first fuel injection in the interval after the closing of the exhaust valve and before top dead centre of a subsequent piston exhaust stroke and to perform a subsequent, first combustion during the negative valve overlap. In addition, the control unit may be arranged to control the fuel injection system so as to perform a second fuel injection before top dead centre of the piston compression stroke and to perform a subsequent, second combustion prior to compression ignition.
In the following text the first injection or injections will generally be referred to as a pilot injection, while any subsequent injection will be referred to as a post injection. The pilot or first fuel injection occurs in the interval between closure of the exhaust valve and top dead centre of the piston exhaust stroke. Said pilot injection may be a single second injection or comprise two or more injections. The total amount of the pilot injection always exceeds the amount injected in the post injection. At least one further fuel injection occurs during the compression stroke, but before top dead centre of said compression stroke. As stated above, the second injection occurs after the pilot injection and is referred to as a post injection. The quantity of the second injection is greater than zero but less than 50% of the total amount of injected fuel.
According to the invention, an efficient triple combustion is generated when the engine is operated at low load conditions in HCCI-mode. In order to achieve this, a first part of the fuel is injected after exhaust valve closing but before TDC of the negative valve overlap, as described above. In this way, a large amount of fuel is combusted, or oxidized in an exothermic reaction, to perform a first combustion step to release heat in the negative valve overlap. Fuel that is not oxidized is cracked in order to auto-ignite easier in the subsequent main compression stroke.
The fuel injection pressure is normally in the order of about 100 bar. When in the post injection mode it could be beneficial to lower the fuel-pressure during the post injection to about 35 bar in order to keep the fuel close to the spark-plug. According to one embodiment, the post injection fuel is preferably, but not necessarily, injected at a lower pressure than that of the pilot injection. According to an alternative embodiment, both the pilot and post injections are performed at a pressure that is lower than said normal pressure. This function may be achieved by using a rate shaping injector controlled by the engine control unit. The pressure reduction level is limited by smoke and NOx emissions. In the hot mixture environment the fuel spray evaporates very fast, whereby small fuel droplets and fuel vapour are pushed away from the spark-plug by the fuel spray initiated air motion. Lowering the fuel pressure reduces the penetration length of the spray and creates larger droplets requiring more time to evaporate, so that the fuel is more concentrated in the vicinity of the spark-plug. It is difficult to lower the fuel pressure within one engine cycle so the pilot and post injections may have the same fuel-pressure. In general, lowering of the fuel pressure gives friction benefits.
According to a further preferred embodiment, the second combustion is a stratified combustion of a relatively small post fuel amount The post fuel amount may be injected early in the compression stroke, typically between 60 to 20 crank angle degrees (CAD) before TDC, and with a spark occurring close to the end of injection to initiate the combustion. In this way the temperature of the mixture being compressed is elevated in addition to the earlier temperature elevation from the first combustion in the negative valve overlap. This can be achieved while generating almost no NOx or particulate emissions. The second combustion is followed by auto-ignition prior to TDC of the compression stroke to complete the triple combustion cycle.
The amount of fuel injected during first and second injection is determined by the control unit on the basis of combustion phasing and a comparison between predetermined limit values for an engine misfire signal and a combustion stability signal transmitted from said at least one sensor. One such sensor may be an ion current sensor. In addition to using control of the amount of fuel injected during first and second injection, the combustion phasing may also be controlled by manipulating the excess oxygen level during negative valve overlap. The excess oxygen level can be controlled by changing the exhaust valve closing timing and/or the intake valve opening timing.
Because of changing engine operation behaviour, the HCCI operational window borders are not fixed, but float, depending on a number of variables indicated by sensors for monitoring various engine operating conditions. The HCCI operational window borders define the limits at which the engine switches from HCCI-mode to SI-mode. Various injection strategies are used within the HCCI operational window to control the combustion phasing, that is the timing of the auto-ignition combustion. In order to maintain stable combustion in HCCI-mode, load threshold values are calculated in order to indicate the size of the combustion phasing control window.
When a sensor detects a combustion phasing close to the earliest possible phasing, then the load threshold value is small for a load increasing demand and large for a load decreasing demand. The earliest possible phasing may be determined by e.g. a knocking threshold or a set-value. In this context, a set-value is a predetermined value for the latest, or earliest, allowed phasing and may be used under conditions when no engine knock signal is generated. For example: a too early phased combustion, albeit very inefficient, may not generate any engine knock during low load conditions. When the control unit and/or a sensor detects a combustion phasing close to the latest possible phasing, then the load threshold value is large for a load increasing demand and small for a load decreasing demand. The latest possible phasing may be determined by e.g. a knocking threshold, a cycle-to-cycle threshold value or a predetermined set-value.
If the load demand exceeds the load threshold value then a mode-switch strategy between HCCI and SI is activated in cases where the operational window borders are crossed. If the load demand exceeds the threshold value without crossing the operational window borders, then the injection strategy is changed. If the demand does not exceed the load threshold value, engine parameters such as fuel quantity are set directly by the engine control unit. The threshold value may be related to a certain percentage of the load. When the injection strategy is determined, the engine parameters such as valve-timing, fuel quantity, timing, spark timing, fuel pressure etc. are set.
Similar to the HCCI operational window borders defined above, the load threshold values between adjacent load regions, or injection strategies, are not fixed, but can be floating depending on load and combustion phasing related variables. These variables may be indicated by sensors for monitoring various engine operating conditions.
Sensors used for determining the knocking or cycle-to-cycle threshold values can be combustion phasing indicators, such as cylinder pressure sensor or indicator, but also derivatives of ion-current measurements, NOx emissions indicating devices, lambda sensors, knock indicating devices or cycle-to-cycle variation indicators.
The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings, and from the claims.